This invention relates to a method for treating a molten metal, such as aluminum or aluminum alloy, to remove trace element impurities and gas and solid impurities therefrom.
Molten metal, such as aluminum, including alloys containing over 50% aluminum, often contains gas and solid impurities, such as dissolved hydrogen and aluminum oxides. Molten aluminum also typically contains alkali and alkaline earth elements such as about 0.002 wt.% Na or 0.001 wt.% Ca, or both. A number of processes have been employed to purify the metal using a gas containing chlorine, such as a mixture of argon and chlorine. Such a process is described in U.S. Pat. No. 3,839,019, incorporated herein by reference. One problem sometimes encountered as processes using chlorine treatment are pressed for increased productivity is that difficulties can be encountered in separating the salts formed as chlorine reaction products, which salts are largely liquid in character. These salts can be difficult to separate and can be carried by the molten aluminum to the casting station and result in surface and subsurface defects in the cast ingot, such as oxide patches which, in turn, can give rise to problems in rolling the ingot into plate or sheet products. Since the oxide patch problem is believed to be associated with the liquid salt reaction products formed by reacting chlorine with metal, such as magnesium, present in the aluminum, it has been proposed to employ reactive fluorine compounds, such as fluorocarbons, since the fluoride reaction products are predominantly solid and do not present the same separation problems as liquid salt products. Hence, fluorocarbons, such as dichlorodifluoromethane (CCl.sub.2 F.sub.2), have been employed in treating molten aluminum with a reactive gas to reduce the amounts of gas impurities and oxides, along with impurity elements such as sodium and calcium. U.S. Pat. No. 3,854,934, incorporated herein by reference, is an example disclosing use of fluorocarbons for treating molten aluminum under a supernatent salt cover. Even though CCl.sub.2 F.sub.2 contains chlorine, the presence of the fluoride salt reaction products tends to tie up the chloride reaction products into fluoride-chloride complexes which behave as solids and are relatively easy to separate from the molten metal. One problem with fluorocarbons, a readily available volatile fluoride source, is that they necessarily contain carbon. While the chlorine and fluorine values are consumed by reacting with impurities in molten aluminum, the carbon reacts with aluminum to form aluminum carbide, which forms an inclusion. Thus, the fluorocarbon treating processes intended to remove trace elements, gas and oxides can tend to do so at the expense of adding an additional impurity; namely, aluminum carbide as an inclusion impurity. This has somewhat hindered acceptance of the fluorocarbon treatment in high volume applications.